Ha ha ha Well I live in Colombia where the rule, pretty much regardless of age, is "let no cleavage go unshown"!
But seriously the thing that is most important and in the end will be the greatest attraction is common interests. However before you can know if there are common interests you have to have contact and that leads back to your question.
Body language is the easiest to spot and there are countless books that explain how we, subconsciously, send messages.
If a woman is sitting and both feet are pointing at you that is a clue, if when you look in her general direction she flips or plays with her hair that is another; and so on. Some are more obvious than others.
If a man flexes when you look his way or pushes his hair back then he is interested.
Just be sure that it is you and no someone behind you that is getting the signal, that can be embarrassing.